In the northwestern Pacific Ocean, scientists have found what they believe to be the biggest volcano on Earth. In fact, to find a volcano of a similar size, you'd have to go to Mars. As NPR's Christopher Joyce reports, the volcano is, fortunately, dormant, but in its prime, it changed the face of the Earth.

CHRISTOPHER JOYCE, BYLINE: William Sager says he brings conversations to a halt when he tells people he's a geophysicist. But now, he says he's got a story that gets people's attention.

WILLIAM SAGER: Where we say, oh, look, there are volcanoes here as big as any that we've seen elsewhere in the solar system. So it's sort of like discovering a new whale or something.

JOYCE: The only bigger one known to science is on Mars, called Olympus Mons. He calls his volcano Tamu Massif. It's about the size of New Mexico. Its top rises 2 1/2 miles, but it doesn't break the surface of the ocean. It's so massive, it caved in the Earth's crust. Sager studies oceanic plateaus and seamounts, the mountain ranges of the oceans. There are thousands of them, but none like this one.

SAGER: You see these little pimples out there, and those are the normal seamounts, and this Tamu Massif just dwarfs them.

JOYCE: The volcano was formed 144 million years ago and is nestled in an underwater plateau. Geoscientist Jackie Caplan-Auerbach at Western Washington University says scientists knew there had been volcanic activity there.

JACKIE CAPLAN-AUERBACH: But we've always thought they were clusters, you know? You would have eruptions at a number of places, and they would all sort of overlap. But this is not. You basically had one place just cranking out enormous amounts of lava.

JOYCE: Sager's team has only surveyed a small part of Tamu Massif, however. It will take further examination to convince the scientific community that it's a single volcano and has earned the title world's biggest. The volcano arose in a section of the Earth's crust where three huge crustal plates come together. How it formed is unclear, possibly when a giant blob of material floated up from inside the Earth, got cooked into magma and burst through the crust. Sager at the University of Houston describes the Tamu Massif in the journal Nature Geoscience. As for the name Tamu, it's the initials of Texas A&M University, where he used to work. Christopher Joyce, NPR News. Transcript provided by NPR, Copyright NPR.